configure/Makefile: Include contrib in "make dist"

Change-Id: I2d6a2e06f075391e8e51d1342a1afe955cc127b0
diff --git a/contrib/etc_default/Makefile.am b/contrib/etc_default/Makefile.am
new file mode 100644
index 0000000..49c9fe6
--- /dev/null
+++ b/contrib/etc_default/Makefile.am
@@ -0,0 +1,3 @@
+EXTRA_DIST = osmo-remsim-bankd \
+	     osmo-remsim-client \
+	     osmo-remsim-client-0 osmo-remsim-client-1 osmo-remsim-client-2 osmo-remsim-client-3